[chromium] Try to get the webkit builder to use msvs again
https://bugs.webkit.org/show_bug.cgi?id=110771

Reviewed by Eric Seidel.

The msvs gyp generator apparently doesn't write the sln file on every
run, so check the vcxproj file instead.

* Scripts/webkitdirs.pm:
(determineIsChromiumNinja):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@143951 268f45cc-cd09-0410-ab3c-d52691b4dbfc